Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery is provided. The lithium ion secondary battery generally comprises an electrode assembly, a container for accommodating the electrode assembly; and an electrolyte. The electrode assembly comprises two electrodes having opposite polarities and a separator. The separator comprises a porous membrane comprising clusters of ceramic particles. The porous membrane is formed by bonding the particle clusters with a binder. Each particle cluster is formed either by sintering or by dissolving and re-crystallizing all or a portion of the ceramic particles. The ceramic particles comprise a ceramic material having a band gap. Each particle cluster may have the shape of a grape bunch or a lamina, and may be formed by laminating scale or flake shaped ceramic particles.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2004-0094067, filed on Nov. 17, 2004, in the KoreanIntellectual Property Office, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to lithium ion secondary batteries, andmore particularly, to electrode assemblies having functional separatormembranes for use in lithium ion secondary batteries.

BACKGROUND OF THE INVENTION

Secondary batteries are rechargeable and can be compact in size and havehigh capacity. Recently, the demand for portable electronic devices,such as camcorders, portable computers, and mobile phones, hasincreased, leading to active research into and development of secondarybatteries. Representative examples of widely-used secondary batteriesinclude nickel metal hydride (Ni-MH) batteries, lithium (Li) polymerbatteries, and lithium ion (Li-ion) batteries.

Since lithium has a small atomic weight, secondary batteries usinglithium can have high capacitance per unit weight. However, sincelithium readily reacts with water, lithium-based secondary batteriesgenerally use non-aqueous electrolytes. In such lithium-based secondarybatteries, there is no water restricting the charging and dischargingvoltage, enabling electromotive forces of about 3 to 4 V to be obtained.Lithium ion secondary batteries are an example of such lithium-basedsecondary batteries.

The non-aqueous electrolytes used in lithium ion secondary batteries aregenerally classified into liquid electrolytes and solid electrolytes.Liquid electrolytes are manufactured by dissolving a lithium salt in anorganic solvent. The organic solvent, for example, may be an alkylcarbonate such as ethylene carbonate or propylene carbonate, or may beanother organic compound.

One problem with these electrolytes, however, is that they typicallyhave low ion conductivity. To compensate for the low ion conductivity ofthese electrolytes, the areas of the electrode activation materials havebeen increased, creating an overlap between the electrodes.

However, increasing the overlap between the electrodes limits batteryperformance in several ways. For example, the low ion conductivity ofthe electrolyte causes high internal impedance of the battery, resultingin an increased voltage drop in the battery. In particular, the low ionconductivity of the electrolyte causes a reduction in battery current,thereby also reducing battery power.

In addition, movement of lithium ions is limited by a separator which ispositioned between the two electrodes. If the separator is notsufficiently permeable and wettable to the electrolyte, the movement oflithium ions is substantially inhibited. As a result, the electricalproperties and performance of the battery may deteriorate.

Therefore, in addition to heat resistance, heat deflection resistance,chemical resistance, and mechanical strength, the cross-sectional poreratio of the separator and the wettability of the separator to theelectrolyte are important characteristics of the separator fordetermining battery performance. The cross-sectional pore ratio of aporous object (e.g., the separator) is the ratio of pore areas to thearea of the cross section.

The separator serves as a safety device for preventing the battery fromoverheating. If the temperature of the separator increases beyond apredetermined level due to some abnormalities in the battery, thepolyolefin-based porous membrane of the separator becomes soft andpartially melts. This causes the pores of the porous membrane to close.The pores function as passages for the electrolyte solution, and morespecifically, for lithium ions. When the flow of lithium ions stops, thecurrent flow between internal and external portions of the batterybecomes blocked, thereby slowing or stopping the temperature increase inthe battery.

However, the temperature in the battery can suddenly and continuouslyincrease for any reason, for example external thermal transfer. When thetemperature increases for more than a predetermined time interval, theseparator may melt and destruct irrespective of the shutdown of thepores. This creates partially melted portions in the separator, andthese partially melted portions may cause the two electrodes of thebattery to be in contact with each other, thereby forming a shortcircuit. Alternatively, the separator may contract, creating contractedportions in the separator. These contracted portions may also cause thetwo electrodes of the battery to be in contact with each other, forminga short circuit. Short circuits cause serious harm to the battery.

In a high capacitance secondary battery, a large amount of current canflow over a short period of time. When excessive current flows in such abattery, the temperature in the battery cannot be decreased by shuttingdown the pores of the separator and blocking current flow. Furthermore,the heat generated by such excessive current flow may cause theseparator to continue to melt and destruct. As a result, a short circuitdue to the destruction of the separator becomes increasingly possible.

Although blocking the current flow by shutting down the pores of theseparator is important, a countermeasure against melting and contractionof the separator is also important to prevent the battery fromoverheating. Therefore, a need exists for a separator which preventsshort circuits between the electrodes at high temperatures, for example,at 200° C. or higher.

SUMMARY OF THE INVENTION

The present invention provides a lithium ion secondary battery having aporous separator membrane which is not easily melted or contracted athigh temperatures, thereby preventing an inter-electrode short circuit.

The present invention also provides a lithium ion secondary batteryhaving a porous separator membrane with high permeability to lithiumions in an electrolyte solution irrespective of changes in temperature.

The present invention also provides a lithium ion secondary batteryhaving good discharge characteristics and long cycle life.

According to one embodiment of the present invention, there is provideda lithium ion secondary battery generally comprising an electrodeassembly, a container for accommodating the electrode assembly, and anelectrolyte. The electrode assembly comprises two electrodes havingopposite polarities, and a separator. The separator comprises a porousmembrane formed by first sintering or dissolving and re-crystallizingceramic particles. These ceramic particles preferably comprise ceramicmaterials having band gaps. The sintering or dissolution andre-crystallization process creates particle clusters which are thenbonded together using a binder. As used, a “band gap” is a difference inenergy between the valence and conduction bands in an insulator orsemiconductor.

In another embodiment of the present invention, each of the particleclusters preferably takes an aciniform shape or a lamina shape. Each ofthe aciniform or lamina shaped particle clusters may be formed bylaminating scale-like particles.

The particle clusters, which comprise ceramic particles, may be formedby various methods including chemical and physical methods. In onechemical method, the ceramic particles are dissolved in a chemicalsolvent. Alternatively, a portion of the surfaces of the ceramicparticles are dissolved. The dissolved ceramic particles are thenre-crystallized to form the particle clusters.

In a physical method, external pressure may be used. In addition, insome methods, the ceramic particles are heated to near their meltingpoint temperature, and then subjected to a necking process.

The amount of ceramic particles used to form the particle clusters mayvary, but preferably enough are used to ensure that the particleclusters take on the desired shapes. Enough of the ceramic particles arealso preferably used to ensure that the particle clusters form a lowdensity porous membrane. This low density porous membrane is formed bymixing the particle clusters with a binder and a solvent and stirringthe mixture to form a porous membrane paste or a dispersive porousmembrane solution.

In yet another embodiment of the present invention, the porous membranemay be coated on the surface of at least one of the two electrodes inthe lithium ion secondary battery. The process for coating the porousmembrane may include applying an activation material to the electrodes,applying the porous membrane solution to the resulting electrode, andbaking the electrode to remove the solvent from the porous membranesolution.

In another embodiment of the present invention, the porous membrane maybe deposited on the entire surface of the electrode plate by firstpreparing a porous membrane solution. The porous membrane solution isformed by uniformly dispersing the particle clusters in a mixed solutioncontaining a solvent and a binder. The electrode plates, which areformed by coating the electrodes with an activation material, are thendipped in the porous membrane solution. Alternatively, the porousmembrane may be deposited on the electrodes by spraying the porousmembrane solution on the electrodes.

In one embodiment of the present invention, the binder material maycomprise a polymer resin. The polymer resin may be an acrylate polymer,a methacrylate polymer, or an acrylate-methacrylate copolymer exhibitingheat resistance at temperatures of about 200° C. or higher.

In yet another embodiment of the present invention, the ceramic materialis selected from the group consisting of zirconium oxide (ZrO₂), alumina(Al₂O₃), silica (SiO₂), titanium oxide (TiO₂), an ion-conductive glass,and combinations thereof. Preferably, the ceramic material compriseszirconium oxide (ZrO₂).

In addition, the ceramic material of the porous membrane preferablyfurther comprises at least one compound selected from the groupconsisting of an insulating nitride compound, a hydroxide compound, aketonic compound of zirconium (Zr), aluminum (Al), silicon (Si) ortitanium (Ti), and mixtures thereof. As used, “an insulating nitridecompound” excludes conductive nitride compounds such as titanium nitride(TiN), which is not suitable for use as the ceramic material inaccordance with the present invention.

As described above, the ceramic material preferably comprises zirconiumoxide. Zirconium oxide is well dispersed in the acrylate binder materialby stirring, as measured by the zeta potential. As a result, theproductivity of the ceramic material can be increased and the costreduced. Furthermore, zirconium oxide exhibits excellent heatdissipation. Also, zirconium oxide can form pn junctions with lithiumcompounds at high temperatures, thereby forming diodes. In addition,zirconium oxide effectively prevents excessive amounts of lithium ionsfrom entering the negative electrode.

In another embodiment of the present invention, each of the ceramicparticles in the aciniform shaped particle clusters preferably has adiameter of about 0.01 μm to about 0.3 μm. Each of the ceramic particlesin the lamina shaped particle clusters preferably takes the shape of ascale-like flake having a width of about 0.1 μm to about 1 μm. Once theparticle clusters having the desired characteristics are formed, ascanning electron microscope (SEM) photograph can be taken to measurethe size of the particles.

In yet another embodiment of the present invention, the ceramic materialpreferably has a thermal expansion rate of 0.2% or less at a temperatureof about 200° C., and a thermal expansion rate ranging from about 0.1 toabout 0.4% at a temperature of about 400° C. If the ceramic material hasa thermal expansion rate higher than 0.2% at about 200° C. or 0.4% atabout 400° C., the internal pressure of the battery increases, whichpressure increase may cause the shape of the battery to deform.

In another embodiment of the present invention, the cross-sectional poreratio of the porous membrane, which includes ceramic particle clustersand a polymer resin binder, is preferably 50% or higher. Thecross-sectional pore ratio indicates the degree of porosity of an objectwith respect to the volumetric pore ratio of that object. Morespecifically, the cross-sectional pore ratio of a porous object isdefined as the ratio of pore areas in a cross section of the object tothe area of the cross section. The volumetric pore ratio is defined asthe ratio of the volume of pores in an object to the volume of theobject. The cross-sectional pore ratio of the porous membrane can bemeasured by first cutting the membrane, and then taking a SEM photographof the cross-section.

As described above, the porous membrane of the present inventioncomprises particle clusters which may be formed by sintering the ceramicparticles. As a result of the sintering process, the particle clustersare irregularly shaped. The irregular shapes of the particle clustersenable the bonding of the clusters to be controlled so as not to fillthe pores created by the irregular shapes of the particle clusters. As aresult of such controlled bonding, fillers comprising the ceramicmaterial also cannot fill the pores in the porous separator membrane. Inaddition, the amount of the binder resin may be reduced, preventinglarge amounts of the binder resin from filling the pores. By thesemethods, the cross-sectional pore ratio of the separator is increased.

In yet another embodiment of the present invention, the ceramic materialpreferably has a thermal conductivity of about 10±0.5 W/(m·K) at atemperature ranging from about 500 to about 1,000° C.

In another embodiment of the present invention, the ceramic materialpreferably has a dielectric constant of about 1 to about 20. If thedielectric constant of the ceramic material is more than about 20,sufficient capacitance cannot be obtained. If the dielectric constant ofthe ceramic material is less than about 1, the material does not have aband gap, and is therefore not useful with the present invention.

In another embodiment of the present invention, the ceramic materialpreferably has a dielectric loss ranging from about 10⁻⁵ to about 10⁻²at a frequency of 1 MHz. If the dielectric loss of the ceramic materialis less than about 10⁻⁵, a predetermined band gap cannot be reproduciblyobtained, making it difficult to manufacture a reliable product. If thedielectric loss of the ceramic material is more than about 10⁻²,sufficient capacitance cannot be obtained.

According to another embodiment of the present invention, a lithium ionsecondary battery generally comprises an electrode assembly, a containerfor accommodating the electrode assembly and an electrolyte. Theelectrode assembly comprises two electrodes having opposite polarities,and a separator positioned between the two electrodes. The separatorcomprises a porous membrane formed by bonding particle clusters with abinder. Each particle cluster is formed by agglomeration of ceramicparticles.

As used, “agglomeration” means spatially gathering, and excludesadherence using binders or other adhesives. In addition, “agglomeration”may include a partial sintering method or a method of dissolving andre-crystallizing all or a portion of the ceramic particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by reference to the following detaileddescription when considered in conjunction with the attached drawings inwhich:

FIG. 1 is a scanning electron microscope (SEM) photograph of particleclusters according to one embodiment of the present invention;

FIG. 2 is a SEM photograph of particle clusters according to anotherembodiment of the present invention;

FIGS. 3 a and 3 b are SEM photographs of scale-like ceramic particlesaccording to one embodiment of the present invention shown at differentmagnifications; and

FIG. 4 is a schematic view of bonded particle clusters according to oneembodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be describedwith reference to the accompanying drawings. An exemplary method formanufacturing an electrode assembly of a lithium ion secondary batteryaccording to one embodiment of the present invention will also bedescribed in detail with reference to the accompanying drawings.

FIGS. 1 and 2 are scanning electron microscope (SEM) photographs ofparticle clusters of zirconium oxide particles. The zirconium oxideparticles constitute the ceramic material of the porous membraneaccording to one embodiment of the present invention. FIGS. 3 a and 3 bare SEM photographs of scale-like ceramic particles shown at differentmagnification rates.

A method for manufacturing an electrode assembly of a lithium ionsecondary battery generally comprises first forming two electrodes byapplying a slurry containing a binder resin and an electrode activationmaterial to a current collector. A porous membrane is then disposed onat least one surface of the two electrodes. The two electrodes arepositioned facing each other in the battery. Finally, the two electrodesare assembled into the electrode assembly such that the porous membraneis disposed between the two electrodes.

The electrode assembly may comprise a jelly-roll type construction,wherein the two laminated electrodes are wound together, and the porousmembrane is disposed on the outer surface of at least one of theelectrodes. In this configuration, when the electrodes are woundtogether, the porous membrane disposed on the surface of at least one ofthe electrodes separates the two electrodes. Alternatively, the porousmembrane may be disposed on the inner surface, or on both the outer andinner surfaces of either one or both of the electrodes.

In an alternative embodiment, the electrodes are dipped in a porousmembrane solution. Such a solution may be formed by uniformly dispersingthe particle clusters in a solution containing a binder and a solvent.The porous membrane can be disposed on the entire surface of eachelectrode. By this construction, it is possible to effectively prevent ashort circuit between the electrode on which the porous membrane isdisposed and the other electrode.

In this embodiment, the electrodes of the secondary battery may beformed according to any known method. For example, an activationmaterial slurry containing a binder and an activation material powder isfirst applied on at least one surface of a metal foil or a metal meshcurrent collector preferably comprising copper or aluminum.

Suitable copper current collectors comprise copper foils havingthicknesses of about 10 to about 15 μm. The activation material slurryis applied on both surfaces of the foil. The activation material slurryis formed by mixing an activation material, such as graphite and activecarbon, with a binder material, such as polyvinylidene fluoride (PVdF).Since the graphite and active carbon are conductive materials, anadditional conductive material is generally not needed.

Suitable aluminum current collectors comprise aluminum meshes havingthicknesses of about 20 μm. The activation material slurry comprisesactivation material powder containing a lithium compound, such aslithium cobalt oxide, lithium peroxide and/or lithium nickel oxide, aconductive material, and a binder. The activation material slurry iscoated on a surface of the aluminum mesh.

In order to form the activation material slurry, a suitable amount ofsolvent, for example, acetone, is used to dissolve the binder. Thesolvent is removed by baking after coating the activation materialslurry.

After coating, the electrode is pressed with a roller. The layer ofactivation material coating has a uniform thickness of about 100 μm.

In this embodiment, the porous membrane is not formed as a separatefilm, but rather is coated on the electrodes. The porous membrane isformed by applying a porous membrane solution or a porous membraneprecursor to at least one surface of at least one of the electrodes onwhich the activation material slurry is coated. If a porous membranesolution is used, the solvent is removed from the porous membranesolution after application to the electrode surface. When a porousmembrane precursor is used, the precursor is cured after application ofthe precursor to the electrode surface.

More specifically, the porous membrane is formed by first preparing asolution containing the binder material, the solvent, and the ceramicpowder. The solution is then applied on the entire surface of thedesired electrode surface by a printing method or a spraying method. Inthis embodiment, the porous membrane is formed by dipping the electrodeinto the solution, adjusting the thickness of the membrane with agravure roller, and baking the resultant electrode.

For example, a powder of particle clusters containing zirconium oxideand an acryl resin comprising methacrylate were mixed at a mixing ratioof 96:4 wt %. The resultant mixture was diluted with a solvent to obtaina solution having a viscosity of 3,000 cps. The surface of the electrodewas coated with the mixture solution by using the dipping method. Theelectrode was dried at a temperature of 120° C. in a drying apparatus,to remove the solvent. As a result, the porous membrane was formed onthe surface of the electrode. This example is presented for illustrativepurposes only, and it is understood that the porous membrane may beformed by various other methods.

The porous membrane according to the present invention is constructed bybonding aciniform shaped particle clusters or lamina shaped particleclusters with a binder to obtain a desired porosity of the membrane. Theaciniform shaped particle clusters are formed by agglomeration ofceramic particles. Each of the ceramic particles preferably has adiameter of about 0.01 to about 0.3 μm. The lamina shaped particleclusters are formed by sintering and laminating a portion of the ceramicparticles, each of which ceramic particles have the shape of a scale orflake. Each of the flake shaped ceramic particles preferably has a widthof about 0.1 μm to about 1 μm. The porous membrane comprising theparticle clusters may have a thickness ranging from about 1.5 to about30 μm.

The particle clusters may be formed either by sintering or by dissolvingall or a portion of the ceramic particles and then re-crystallizing theresulting product. When the ceramic particles are dissolved andre-crystallized, they agglomerate, forming the particle clusters, whichare then obtained by extraction.

The porous membrane solution is then formed by first mixing a powder ofthe particle clusters with a binder and a solvent. If the bindersurrounds the entire surface of each particle cluster, and the particleclusters are bonded to each other with the binder, and if the ionconductivity of the binder is low, then the ion conductivity of theparticle clusters and, consequently, of the porous membrane, may be low.In order to ensure a high ion conductivity of the porous membraneirrespective of the ion conductivity of the binder, the binder ispreferably disposed on only a portion of each particle cluster 10,forming bridge-like bonds 12, as shown in FIG. 4. As shown, the particleclusters 10 are connected by the bridge-like bonds 12, thereby ensuringhigh ion conductivity of the porous membrane.

According to the present invention, a slurry used for forming the porousmembrane preferably includes a small amount of binder material. Theweight ratio of the ceramic material and the binder material in theporous membrane preferably ranges from about 98:2 to about 85:15 toprevent the binder from entirely surrounding the particle clusters. As aresult, it is possible to prevent the reduction in ion conductivity ofthe fillers due to the surrounding binder.

In one embodiment, the particle clusters are formed by partiallysintering the ceramic particles. This partial sintering processcomprises heating the ceramic particles, such as zirconium oxide, at atemperature of about 900° C. for about 10 minutes. The partiallysintered ceramic material is then completely dissolved in a solventcapable of dissolving the ceramic material. Alternatively, the powder ofthe partially sintered ceramic particles is partially dissolved in asolvent. The partially sintered ceramic particles are then re-extractedby removing the solvent, thereby forming the particle clusters.

In the porous membrane according to the present invention, there arepores in the ceramic particles, there are pores between the ceramicparticles in the particle clusters, and there are pores provided by thebinder between the particle clusters. Each of these pores increases thecross-sectional pore ratio of the porous membrane. These pores alsoincrease the permeability of the porous membrane to the electrolytesolution and increase the ion conductivity of the porous membrane to thelithium ions.

In the present invention, the cross-sectional pore ratio of the porousmembrane is preferably 50% or more. In order to ensure that thecross-sectional pore ratio of the porous membrane is 50% or more, thebinder is preferably present in the porous membrane in an amount of 20%or less. In addition, the binder is not expansible. More specifically,the binder has a low expansibility to the electrolyte solution.Non-limiting examples of suitable binders for use with the presentinvention include acrylate resin binders. In addition to having lowexpansibility to the electrolyte solution, acrylate resin binders haveexcellent adhesiveness.

In addition, the binder is uniformly dispersed, thereby reducing theamount of binder needed. However, the process for uniformly dispersingthe binder is very difficult. Accordingly, a porous membrane having ahigh cross-sectional pore ratio is difficult to obtain. In the presentinvention, the partially sintered ceramic particle clusters are used asfillers so that the acrylate resin binder is easily dispersed. As aresult, a porous membrane having a high cross-sectional pore ratio isobtained with a small amount of binder material.

The two electrodes, on at least one of which the porous membrane iscoated, are wound together. Since the porous membrane serves aseparator, it is not necessary to provide an additional separatorbetween the two electrodes. In this configuration, although there is noseparator to shut down in order to block current flow when the batteryis overcharged or overheated, the porous membrane functions to block theion flow. More specifically, when the battery is overcharged oroverheated, lithium ions are extracted by the portions of the porousmembrane that are poorly adhered to the electrodes. Therefore, althoughthe electrodes may be in contact with each other at the points where theporous membrane is poorly adhered to the electrodes, the extraction oflithium ions by the porous membrane blocks the current flow. Inaddition, when the battery is overheated, a pn junction diode-likestructure is formed between the ceramic material and thelithium-containing material. Therefore, the porous membrane, like aseparator, functions to block the current flow, thereby ensuring thesafety of the battery.

In addition, an insulating film comprising a polyolefin resin may beprovided between the two electrodes. The polyolefin resin insulatingfilm together with the porous membrane serve as the separator in thelithium ion secondary battery.

The inventive method for manufacturing the separator by disposing aporous membrane on the surface(s) of the electrode has severaladvantages over conventional methods which use an additional separator.

In the conventional methods which use an additional separator, theseparator is laminated on the two electrodes and the resulting productis wound to form an electrode assembly. According to these conventionalmethods for forming the electrode assembly, the separator and theelectrodes may be misaligned, increasing the possibility of a shortcircuit between the electrodes. However, no misalignment occurs in theformation of the electrode assembly according to the present inventionbecause the separator is coated on the electrodes and is formed togetherwith the electrodes. Therefore, the method according to the presentinvention does not require an alignment process.

In addition, when the battery is overheated, the separator used in theconventional methods may become contracted, increasing the possibilityof a short circuit between the electrodes. Because the separatoraccording to the present invention is coated on the electrodes, theseparator does not contract due to the adhering force between theseparator and the electrode. Therefore, a short circuit cannot occurbetween the electrodes when the battery is overheated. In addition,unlike the battery prepared according to the conventional methods, inthe battery according to the present invention, the possibility ofdamaging the battery during manufacturing is reduced.

Just like the electrical resistance of a resistor is inverselyproportional to the length of the resistor, the ion conductivity of theseparator is inversely proportional to the thickness of the separator.Accordingly, the thinner the separator, the greater the battery power.According to the present invention, a thin separator can be formed,making it possible to manufacture a battery with increased power. Inaddition, by adhering the porous membrane to the surface of theelectrode(s), the separator according to the present invention isintegral with the electrode. Therefore, the separator need not have highmechanical strength to endure stress during battery manufacturing.Accordingly, the separator can comprise a flexible material withoutlimiting the strength of the separator.

In the present invention, silica, aluminum oxide (α-alumina) andzirconium oxide are preferably used as the ceramic particles of theporous membrane. Since these ceramic materials are not substantiallydeteriorated at high temperatures, porous membranes comprising theseceramic materials cannot be contracted or melted at high temperatures,for example, temperatures of about 300° C. Therefore, a short circuitbetween the electrodes is prevented. In addition, these ceramicmaterials can increase the stability of the separator. Also, thepartially sintered particle clusters of the porous membrane increase thecross-sectional or volumetric pore ratio of the porous membrane.

A lithium ion secondary battery according to the present invention isformed by placing the electrode assembly formed according to the abovedescribed methods into a can-type container. The can-type container ishermetically sealed with a cap assembly. The electrolyte solution isinjected through an injection hole in the cap assembly, and theinjection hole is then sealed. According to the present invention, thecross-sectional pore ratio of the porous membrane in the electrodeassembly is high. Therefore, the rate of injection of the electrolytesolution is much faster than, and can be twice as fast as, that of theconventional methods using polyolefin separators.

The partially sintered ceramic materials of the present inventionincrease the heat resistance of the separator, thereby preventing damageto the separator and preventing a short circuit between the electrodesupon overheating of the battery.

In addition, the bonding of the partially sintered ceramic particleclusters with the small amount of binder according to the presentinvention increases the ion conductivity of the battery, as well as thecapacitance and efficiency of the battery.

In addition, because the separator according to the present invention isintegral with the electrode(s), misalignment does not occur between theseparator and the electrodes. Also, the separator according to thepresent invention can be thin, thereby increasing the amount ofdischarged activation material and increasing the electrical capacitanceof the battery.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structure may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention. Accordingly, the foregoing description should not beread as pertaining only to the precise structures described andillustrated in the accompanying drawings, but rather should be readconsistent with and as support for the following claims which are tohave their fullest and fairest scope.

1. A lithium ion secondary battery comprising: an electrode assemblycomprising: first and second electrodes having opposite polarities, anda separator comprising a porous membrane, the porous membrane comprisingclusters of ceramic particles and a binder, wherein the ceramicparticles comprise a ceramic material having a band gap, the ceramicparticles are present in an amount greater than an amount of the binder,and the porous membrane has a cross-sectional pore ratio of about 50% orgreater, wherein the binder comprises a polymer resin selected from thegroup consisting of acrylate polymers, methacrylate polymers andacrylate-methacrylate co-polymers and having heat resistance at atemperature of about 200° C. or greater; a container for accommodatingthe electrode assembly, wherein the first and second electrodes arepositioned facing each other; and an electrolyte.
 2. The lithium ionsecondary battery according to claim 1, wherein each of the clusters ofceramic particles has a shape selected from the group consisting ofaciniform and laminae.
 3. The lithium ion secondary battery according toclaim 1, wherein the ceramic material is selected from the groupconsisting of silica (SiO₂), alumina (Al₂O₃), zirconium oxide (ZrO₂),titanium oxide (TiO₂) and mixtures thereof.
 4. The lithium ion secondarybattery according to claim 1, wherein the ceramic material is selectedfrom the group consisting of insulating nitride compounds of silicon(Si), hydroxide compounds of silicon (Si), alkoxy compounds of silicon(Si), ketonic compounds of silicon (Si), insulating nitride compounds ofaluminum (Al), hydroxide compounds of aluminum (Al), alkoxy compounds ofaluminum (Al), ketonic compounds of aluminum (Al), insulating nitridecompounds of titanium (Ti), hydroxide compounds of titanium (Ti), alkoxycompounds of titanium (Ti), ketonic compounds of titanium (Ti),insulating nitride compounds of zirconium (Zr), hydroxide compounds ofzirconium (Zr), alkoxy compounds of zirconium (Zr), ketonic compounds ofzirconium (Zr), and mixtures thereof.
 5. The lithium ion secondarybattery according to claim 1, wherein the separator further comprises apolyolefin-based resin membrane.
 6. The lithium ion secondary batteryaccording to claim 1, wherein the separator consists essentially of theporous membrane.
 7. The lithium ion secondary battery according to claim1, wherein the porous membrane is disposed on at least one surface of atleast one of the first and second electrodes.
 8. The lithium ionsecondary battery according to claim 7, wherein the porous membrane isdisposed on the at least one surface of the at least one electrode bydipping the electrode in a solution comprising a solvent, a binder andthe clusters of ceramic particles.
 9. The lithium ion secondary batteryaccording to claim 7, wherein the porous membrane is disposed on the atleast one surface of the at least one electrode by spraying a solutioncomprising a solvent, a binder and the clusters of ceramic particles onthe at least one electrode.
 10. The lithium ion secondary batteryaccording to claim 7 wherein the porous membrane is disposed on the atleast one electrode by printing a solution comprising a solvent, abinder and the clusters of ceramic particles on the entire surface ofthe electrode.
 11. The lithium ion secondary battery according to claim2, wherein each of the particle clusters has the aciniform shape, andeach of the ceramic particles in the cluster has a diameter of about0.01 to about 0.3 μm.
 12. The lithium ion secondary battery according toclaim 2, wherein each of the particle clusters has the shape of alamina, and each of the ceramic particles has a shape of a scale orflake having a width of about 0.1 μm to about 1 μm.
 13. The lithium ionsecondary battery according to claim 1, wherein the porous membrane hasa thickness of about 1.5 to about 30 μm.
 14. The lithium ion secondarybattery according to claim 1, wherein the ceramic material has a thermalexpansion rate of about 0.2% or less at a temperature of about 200° C.,and a thermal expansion rate of about 0.1 to about 0.4% at a temperatureof about 400° C.
 15. The lithium ion secondary battery according toclaim 1, wherein the ceramic material has a thermal conductivity rangingfrom about 10+0.5 W/(m·K) to about 10−0.5 W/(m·K) at a temperatureranging from about 500 to about 1,000° C.
 16. The lithium ion secondarybattery according to claim 1, wherein the ceramic material has adielectric loss of about 10⁻⁵ to about 10⁻² at a frequency of about 1MHz.
 17. The lithium ion secondary battery according to claim 1, whereinthe weight ratio of the ceramic material to the binder in the porousmembrane ranges from about 98:2 to about 85:15.
 18. The lithium ionsecondary battery according to claim 1, wherein each particle clustercomprises at least three ceramic particles.